1. Field of the Invention
This invention relates to an etching process, a color selecting mechanism for a color cathode-ray tube and a method of manufacturing the same. The invention is applicable to, for instance, the color selecting mechanism for a color television receiver picture tube.
2. Description of Related Art
The color picture tube usually has a color selecting mechanism which faces the color phosphor screen. The color phosphor screen provides color when predetermined color phosphors are illuminated by electron beams corresponding to individual colors R, G and B sorted by the color selecting mechanism.
As the color picture tube color selecting mechanism, various structures have been proposed. For example, a "Trinitron" (a registered trade name) color picture tube uses an aperture grill type color selecting mechanism assembly 100, as shown in the perspective view of FIG. 11, which is disposed such that it faces a color phosphor screen 200. The aperture grill type color selecting mechanism assembly 100 comprises a a thin metal sheet 110 with a number of parallel slits 130 formed therein such that electron beams pass through these slits.
The color phosphor screen 200 comprises vertical color stripes (not shown) of red, green and blue colors which are arranged parallel in a predetermined sequence. The color selecting mechanism assembly 100 is disposed such that it faces the color phosphor screen 200. The thin metal plate 110 has many electron beam passage slits 130 extending in the direction of the phosphor stripes at least from the upper edge to the lower edge of the effective screen area.
Specifically, the thin metal sheet 110 of the aperture grill type color selecting mechanism assembly 100, is made from a high purity steel sheet which is 0.08 to 0.15 mm thick and has a number of parallel electron beam passage slits 130. The thin metal sheet 110 is stretched over a frame 140.
The frame 140 comprises, for instance, a pair of, i.e., an upper and a lower, frame portions 140A and 140B and a pair of arm portions 140C and 140D connecting the frame portions 140A and 140B to each other. The front end surfaces of the frame portions 140A and 140B are curved surfaces forming the same cylindrical surface. As shown in FIG. 1, the thin metal sheet 110 is stretched on the front end surfaces of the frame portions 140A and 1408.
The thin metal sheet 110 is mounted in the frame 140 by using turnbuckles (not shown) for pulling the frame portions 140A and 1408 of the frame 140 toward each other. In this state, edges of the thin metal sheet 110 are welded to the front end surfaces of frame portions 140A and 140B. Then, the turnbuckles are removed to release external forces applied to the frame 140. Thus, by the restoring force of the frame 140 the thin metal sheet 110 is stretched with a predetermined tension generated in the direction of the slits 130.
As a method of forming the electron beam passage slits 130 in the thin metal sheet 110, the following technique is well known in the art.
FIGS. 12A to 12C illustrate a prior art method of manufacturing an aperture grill type color selecting mechanism. The method is suited for manufacturing a color selecting mechanism, with electron beam passage slits arranged at a comparatively coarse pitch, and it is commonly called a single-step double-side etching process.
(Step 10A)
In this single-step double-side etching process, a photosensitive etching resist is coated on both the front and back surfaces of thin metal sheet 110 for forming electron beam passage slits therein. The thin metal sheet 110 is made of iron or a metal composition mainly composed or iron. Subsequently, a first and a second photosensitive etching resist layer 120 and 122 formed on the front and back surfaces, respectively, of the thin metal sheet 110 are patterned. For the sake of brevity, hereinafter the surface of the thin metal sheet 110 formed with the first photosensitive etching resist layer 120 may sometimes be referred to as front surface, and the surface of the thin metal sheet 110 formed with the second photosensitive etching resist layer 122 as back surface.
For the patterning of the first and second resist layers 120 and 122, a pair of photosensitive etching resist masks with respective patterns are used for the front and back surfaces. The pair resist masks have to be positioned stringently. The first and second resist layers 120 and 122 are patterned in a usual method comprising successive steps of exposure, development, drying and hardening. As a result, a pattern with slit-like openings having a width w.sub.1 ' are formed in the first resist layer 120, and a pattern with slit-like openings having a width w.sub.2 ' in the second resist layer 122 (see FIG. 12A). The openings in the resist layers 120 and 122 are formed such that the center line of each opening in the first layer 120 is aligned to or stringently parallel to the center line of the corresponding opening in the second layer 122.
(Step 20A)
Then, with the first and second photosensitive etching resist layers 120 and 122 with the slit-like openings with the widths W.sub.1 ' and W.sub.2 ' as etching masks, the thin metal film 110 is wet etched simultaneously from both the front and back surfaces by using, for instance, an aqueous solution of mercuric chloride. The etching of the thin metal sheet 110 proceeds through the openings with the widths w.sub.1 ' and w.sub.2 ', and eventually electron beam passage slits 130 penetrating the thin metal sheet 110 are formed (see FIG. 12B).
(Step 30A)
Subsequently, the first and second photosensitive etching resist layers 120 and 122 are removed from the surfaces of the thin metal sheet 110. In this way, a color selecting mechanism comprising the thin metal sheet 110, which has a structure as shown schematically in the fragmentary sectional view of FIG. 12C, can be obtained.
The width of the slits 30 on the electron beam incidence side (corresponding to the back surface of the thin metal sheet 110) is defined by the width w.sub.1 ' of the openings formed in the second resist layer 122, while the width of the slits 130 on the electron beam emission side (corresponding to the front surface of the thin metal sheet 110) is defined by the width w.sub.2 ' of the openings formed in the first resist layer 120.
The reason for forming the electron beam passage slits 130 having the sectional profile as described above, will now be described with reference to fragmentary sectional views of FIGS. 13A to 13C showing the thin metal sheet 110. Each electron beam passes through each electron beam passage slit 130 from the electron gun side of the thin metal sheet 110 to the phosphor screen side. The angle of electron beam incidence with respect to the color selecting mechanism is changed in dependence on the position of the electron beam incidence on the color selecting mechanism. If the side walls of the slits 130 are the nearer the vertical, the more the electron beam incident on each slit 130 is subject to reflection by the side walls of the slit 130, as shown in FIG. 13C. When such an electron beam arrives at the phosphor screen, picture tube characteristic reduction results from the reflection of the electron beam (or halation).
On the other hand, with the electron beam passage slits 130 having the sectional profile as shown in FIG. 13A or 113B, the electron beam incident on each slit 130 is less subject to reflection by the side walls of the slit 130. Thus, it is possible to effectively prevent the reflection of electron beam (or halation). For the above reason, usually the area of the slits 130 of the thin metal sheet 110 on the phosphor screen side is made greater than the area of the slits on the electron gun side.
The single-step double-side etching process has a problem that it is difficult to obtain stringent mutual positioning of the front and back surface resist masks. In addition, it is difficult to adequately control the conditions of etching of the then metal sheet 110 from the front and back surfaces thereof. Therefore, it is difficult to obtain a color selecting mechanism having a predetermined sectional profile, that is, having a stable quality.